Antigenic cell surface modulation associated with malignant transformation is being investigated in order to obtain a clear distinction between the normal, preneoplastic, and neoplastic mammary epithelial cells. Xenogenic antibodies were prepared to normal epithelial cell-specific differentiation antigens and to the mammary tumor virus. After appropriate absorptions, these antibodies will be used in immunofluorescence studies to determine the presence and location (plasma membrane, cytoplasm, nucleus, etc.) of the respective antigens. The interrelationship of tumor-associated antigens with normal cell surface antigens (i.e., H-2) will be studied in mammary tumor epithelial cells, preneoplastic (HAN) epithelial cells, and normal mammary epithelial cells. Thus, it is hoped that a correlation between antigenic expression and neoplastic transformation will be found.